The invention pertains to a gear drive with at least a first gear wheel on a first shaft and a second gear wheel on a second shaft a certain distance away from the first shaft. The second gear wheel engages with the first gear wheel by way of helical gearing. A third gear wheel is provided a certain axial distance away from the second gear wheel to compensate for backlash. The third wheel also engages with the first gear wheel by way of helical gearing and can be shifted axially with respect to the second gear wheel.
For a gear drive between two shafts, the distance between the shafts should be planned so that the backlash is neither too large nor too small, because otherwise the gear wheels will either jam or chatter. Especially in the case of shafts with high threshold torques or alternating torques such as the crankshafts of internal combustion engines, the teeth of the meshing gears of a secondary drive must be of high quality and have minimal backlash, because otherwise running noise will be produced by the change from one set of contact surfaces to the other in the area where the teeth mesh. This problem is exacerbated by differences between the thermal expansion of the materials of the gear wheels, which are usually fabricated of steel, and the thermal expansion of the shaft housing, which, in the case of internal combustion engines, is produced out of, for example, aluminum. The difference with respect to thermal expansion causes a change in the distance between the shafts and therefore a change in the backlash. This can be disadvantageous because of the noise which is produced while the internal combustion engine is warming up.
A gear stage with backlash elimination is known from DE 4,326,155 A1. To compensate for changes in backlash, a gear wheel can be shifted in the axial direction. A piston-cylinder arrangement is used to shift the gear wheel hydraulically. A system for compensating in this way for the changes in backlash caused by thermal expansion, however, is complicated in operation and expensive to produce.
The present invention is based on the task of providing a gear drive of the type indicated above which overcomes the disadvantages described above and which can easily and precisely compensate for the changes in backlash caused by differences in the thermal expansion of the gear wheels and the gear wheel housing.
It is therefore provided according to the invention that an expansion element is used to adjust the distance between the second and the third gear wheels as a function of temperature. The thermal expansion behavior of this expansion element is selected so that, when the expansion element undergoes thermal expansion, it will shift the third gear wheel in the axial direction and thus, under consideration of a helical gearing effect, compensate for the play caused by the thermally induced change in the distance between the first and the second shafts.
This has the advantage that, once the predetermined amount of backlash has been set, it will remain constant despite changes in temperature, because the effect of temperature itself brings about the required compensation automatically, without the need for any complicated hydraulic control. When the design according to the invention is used, it is possible for the gear teeth to have relatively small helix angles, which means that high torsional strength is obtained when the teeth are meshing without backlash.
It is advisable for the third gear wheel to be seated on the second shaft by sets of spur teeth, for example, so that it is able to shift in the axial direction. As an alternative, the sliding seat between the second shaft and third gear wheel can be designed with helical teeth, which improves the compensation of the temperature-caused change in backlash.
In a preferred embodiment, the elastic means is a disk spring, and the second shaft is advisably threaded, so that a nut can be screwed onto it. Thus the disk spring can be held in place between the nut and the third gear wheel.
The expansion element can be designed as an expansion ring, for example.
In a preferred embodiment, an elastic means is provided, which acts on the third gear wheel, pushing it axially in the direction of the second gear wheel. The expansion element expands to compensate for changes in backlash in opposition to the force exerted by this elastic means.
In an alternative, preferred embodiment, the second shaft is designed as a hollow shaft, and the expansion element is mounted in the hollow shaft, the hub of the third gear wheel being supported radially in the hollow shaft with play.
In a preferred elaboration of the invention, an expansion rod is connected detachably to the second shaft by a collar. The collar can be provided with several slots, for example, and can be held in place on the second shaft by a screw with a conical clamping section.
It is advisable for the two gear wheels to be mounted at different ends of the second shaft and to mesh with two individual gear wheels of the first shaft.
To save space and to reduce the number of parts, at least one of the two gear wheels of the first shaft is designed as a sensor wheel for an engine control system. It is advisable for the gear wheel of the first shaft serving as the sensor wheel to have at least one missing tooth.